1. Field of the Invention
The present invention relates to a method for controlling the flow rate of an oxidizer in a fuel cell system.
Priority is claimed on Japanese Patent Application No. 2002-344034, filed Nov. 27, 2002, the content of which is incorporated herein by reference.
2. Description of Related Art
Among fuel cells mounted in fuel cell powered vehicles or the like, a type of fuel cell is known in which a solid polymer electrolyte membrane and anode and cathode electrodes that together sandwich the solid polymer electrolyte membrane are provided, and a fuel gas (e.g., hydrogen gas) is supplied to the anode electrode, and an oxidizing gas (e.g., oxygen or air) is supplied to the cathode electrode so that electrical energy is generated through an oxidation-reduction reaction of the fuel gas and the oxidizing gas.
A fuel cell system incorporating the above type of fuel cell is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 07-249421, in which the pressure difference between the anode area and the cathode area of the fuel cell is made controllable depending on the flow rate of the gas supplied to the cathode electrode and the output electrical power.
In such a fuel cell system, when air must be supplied under conditions of high flow rate and low pressure, or under conditions of small flow rate and high pressure, during a transitional state in which operation conditions of the fuel cell system vary, an excessive load is applied on the air compressor of the fuel cell system, and the electrolyte membrane is placed under severe conditions because the pressure difference cannot be controlled, which are not preferred in terms of maintaining reliability of the fuel cell.
In view of the above circumstances, an object of the present invention is to provide a method for controlling the flow rate of an oxidizer in a fuel cell system by which reliability of the fuel cell can be maintained even when the fuel cell system is placed in a transitional state in which operation conditions of the fuel cell system vary.
In order to achieve the above object, the present invention provides a method for controlling the flow rate of an oxidizer in a fuel cell system having a fuel cell stack performing power generation by being supplied with fuel and the oxidizer, a compressor for supplying the oxidizer to the fuel cell stack, a back pressure valve for controlling pressure of the oxidizer which is disposed in a flow passage of the oxidizer and downstream of the fuel cell stack, and a control device for controlling the fuel cell stack, the compressor, and the back pressure valve, the method including the steps of: calculating an oxidizer pressure command and an oxidizer flow rate command based on a given electrical current command using the control device; comparing a first flow rate that is defined as an upper limit of oxidizer flow rate corresponding to the oxidizer pressure command and a second flow rate that is defined as a lower limit of oxidizer flow rate corresponding to the oxidizer pressure command with the oxidizer flow rate command; and regulating the oxidizer flow rate command so as to be limited within a range from the second flow rate to the first flow rate.
According to the above method for controlling the flow rate of an oxidizer in a fuel cell system, the flow rate of the oxidizer is limited within the range from the first flow rate to the second flow rate, i.e., within a range which is easily achievable by the compressor of the fuel cell system even when the operation state of the fuel cell system changes and pressure and flow rate requirements of the oxidizer to be supplied to the fuel cell stack are changed; therefore, the compressor operates under mild conditions and the function thereof is maintained. Moreover, because the pressure difference between the anode area and the cathode area of the fuel cell unit can be controlled so as to be limited within an appropriate range, the electrolyte membrane of the fuel cell unit can be preferably protected, and reliability of the fuel cell stack can be improved.
In the above step of controlling the oxidizer flow rate command, the oxidizer flow rate command may be corrected to the first flow rate when the oxidizer flow rate command is greater than the first flow rate, and the oxidizer flow rate command may be corrected to the second flow rate when the oxidizer flow rate command is less than the second flow rate.